1. Field of the Invention
The present invention relates to an optical information recording-reproducing apparatus which records or reproduces information optically on an optical recording medium like an optical disk. In particular, the present invention relates to an optical information recording-reproducing apparatus in which spherical aberration caused by thickness variation of a cover layer of an optical recording medium is compensated. The present invention relates also to a method of compensation of the spherical aberration in the optical information recording and reproduction.
2. Description of the Related Art
An optical high-density recording process is commercialized which uses a blue-violet semiconductor laser of a wavelength of about 407 nm and an objective lens of NA=0.85. In such a process, being different from processes employing a CD or a DVD, a slight error in the thickness of the cover layer of the optical disk will cause a great spherical aberration to make instable the recording and reproduction. This spherical aberration should be compensated.
For example, Japanese Patent Application Laid-Open No. 2002-312971 discloses a method of compensating the spherical aberration by adjusting the divergence angle of a light beam introduced to the objective lens by driving a collimator lens or a beam expander lens in the optical axis direction.
The spherical aberration can be optimized by adjusting the amplitude of a push-pull signal or an RF signal. However, when the spherical aberration is greater, the focusing is difficult and the optimization of the spherical aberration is prevented. For compensation of the spherical aberration, the initially set standard position of the lens (of the movable part) should be detected.
Japanese Patent Application Laid-Open Nos. 2003-91847 and 2003-131113 disclose use of a photo-interrupter for detecting the standard position. FIG. 10 illustrates the constitution for the detection. As illustrated in FIG. 10, stepping motor 122 drives concave lens 116a of expander lens system 116 through leading screw mechanism 123 and rack 124. With the photo-interrupter fixed on holder 125a of concave lens 116a kept apart from photosensor 128, the light beam emitted from the LED in photosensor 128 reaches the photodiode without interruption of the light beam. In this state, the output from the photosensor is at a high level. On the other hand, with the photo-interrupter placed between the LED and the photodiode, the light emitted by the LED is interrupted to lower the output level of photosensor 128 to a low level. From the output level, the standard position of the movable part can be detected.
Japanese Patent Application Laid-Open No. 2005-327396 discloses a method of detection of a divergence angle of the light beam by separating a part of the light beam penetrating through a beam expander (first and second collimator lenses), in place of mounting a position-detecting member.
The detection of the position of the lens according to the aforementioned Japanese Patent Application Laid-Open Nos. 2002-312971, 2003-91847, and 2003-131113 has disadvantages of a larger size of the apparatus due to a larger number of the constituting parts and the increase of the optical paths. For example, for use of the photo-interrupter, photo-interrupting parts and a photosensor are required.
The detection of a divergence angle by separating partly the light beam emitted from a beam expander as disclosed in Japanese Patent Application Laid-Open No. 2005-327396 requires a light-condensing optical system for condensing the separated light beam onto the detection sensor, and an optical path to the detection sensor in addition to the optical path for reading the information, resulting in a larger size of the optical system, disadvantageously.